METHOD FOR MAKING DEFORMED SEMI-FINISHED PRODUCTS FROM ALUMINIUM ALLOYS

Abstract

 The invention relates to the field of metallurgy and can be used for making deformed semi-finished products in the form of profiles of variously shaped cross-section. A method for making a deformed semi-finished product from an aluminium alloy is provided, comprising the following steps: a) preparing a melt comprising iron and at least one element selected from the group consisting of zirconium, silicon, magnesium, copper and scandium; b) producing a cast blank of infinite length by crystallising the melt at a cooling rate that provides for forming a cast structure characterised by a dendritic cell size of up to 60 µm; c) producing a deformed semi-finished product of final or intermediate cross section shape by hot rolling the blank at an initial temperature of up to 520°C with a degree of reduction of up to 60%, and performing at least one further step comprising pressing the blank at a temperature ranging from 300 to 500°C by passing the blank through a swage; quenching in water the deformed semi-finished product from the previous step at a temperature of no lower than 450°C. The structure of the deformed semi-finished product represents an aluminium matrix with at least one selected doping element and eutectic particles distributed therein and a crosssectional size of up to 3 µm. The method provides for an altogether high level of physical and mechanical properties, in particular, a high degree of relative elongation (of at least 10%) and temporary tensile strength, and a high level of conductivity achieved in one technological stage of manufacturing.

Description

Pertinent art

[0001] The invention relates to metallurgy and can be used to produce deformed semi-finished products as shapes of various cross-sections, rods, rolled sections, including wire rod, and other semi-finished products from technical-grade aluminium and technical-grade aluminium-based alloys. Deformed semi-finished products can be used in electrical engineering to produce wiring products, welding wire, in construction, and for other applications.

Prior art

[0002] Different methods for producing deformable semi-finished products are used to produce products from wrought aluminium alloys and, all other things being equal, such methods determine the final level of mechanical properties. At the same time, it is not always possible to achieve an aggregate high level of various physical and mechanical characteristics, in particular, when high strength properties are achieved, a low plasticity is usually present and vice versa.

[0003] The most common method for producing aluminium wire rod includes such steps as continuous casting of a casting bar, its rolling to produce wire rod, and subsequent coiling of the wire rod. The method is widely used for the production of electrical wire rod, in particular, from technical-grade aluminium, Al-Zr alloys, and 1xxx, 8xxx, and 6xxx-group alloys. The major producers of this type of equipment are VNIIMETMASH (http://vniimetmash.com) and Properzi (http://www.properzi.com). The main advantage of this equipment is first of all the high output in the production of wire rod. Among the disadvantages of this method, one should mention the following:

1) a rolling deformation method does not allow producing geometrically complicated products (in particular, angle sections and other semi-finished products with an asymmetric cross-section);

2) when only a rolling method is used, it is usually not possible to achieve high percentage of elongation and an additional thermal processing is needed to increase the percentage of elongation.

[0004] In addition, during one hot-rolling cycle it is usually impossible to carry out large single-time deformations, which requires to consecutively identify deformation zones, in particular, to use cluster mills, and this will require allocating large production areas for placing the equipment.

[0005] There is another method for producing aluminium alloys, which is reflected in Alcoa patent US20130334091A1. The continuous strip casting and thermal processing method includes the following basic operations: continuous strip casting, rolling to get final or intermediate strips, and further hardening. In order to achieve characteristics of a given level, the proposed method provides for the mandatory thermal processing of deformed semi-finished products, in particular, rolled strip, which, in some cases, complicates the production process.

[0006] The closest to the claimed invention is a method for producing wire, as reflected in patent US3934446. The method involves the continuous wire production process using the following combined steps: rolling of a casting bar and its subsequent pressing. Among the disadvantages of the proposed invention, one should note that there are no process parameters (casting bar temperature, degrees of deformation, etc.) that can ensure the achievement of the required physical and mechanical characteristics.

Disclosure of the invention

[0007] The objective of the invention is to create a new method for producing deformable semi-finished products, which would provide the achievement of an aggregate high level of physical and mechanical characteristics, in particular, high percentage of elongation (minimum 10%), high ultimate tensile strength, and high conductivity, when wrought aluminium alloys alloyed with iron and at least an element of the group consisting of zirconium, silicon, magnesium, nickel, copper, and scandium are used.

[0008] The technical result is the solution of the problem, which is the achievement of an aggregate level of physical and mechanical characteristics in one production stage, excluding multiple production stages, such as separate coil production, hardening, or annealing stages.

[0009] The solution to the problem and the achievement of the technical result mentioned are ensured by the fact that the authors have proposed the method for producing deformed semi-finished products from an aluminium-based alloy, which consists of the following steps:

a) preparing a melt containing iron and at least an element of the group consisting of zirconium, silicon, magnesium, nickel, copper, and scandium.

b) producing a continuous casting bar by crystallisation of the melt at a cooling rate that provides the formation of a cast structure characterised by a dendritic cell size of not more than 70 µm.

c) producing a deformed semi-finished product with a final or intermediate cross-section by hot rolling of the casting bar, with an initial casting bar temperature being not higher than 520°C and a degree of deformation being of up to 60% (optimally up to 50%), and additionally using at least one of the following operations:

• pressing of the casting bar in the temperature range of 300-500°C by passing of the casting bar through the die;
• water quenching of the resulting deformed semi-finished product at a temperature not lower than 450°C.

[0010] In this case, the deformed semi-finished product structure is an aluminium matrix with some alloying elements and eutectic particles with a transverse size of not more than 3 µm that are distributed therein.

[0011] In particular case, rolling can be carried out at a room temperature (about 23-27°C).

[0012] Press-formed products can be rolled by passing them through a number of rolling mill stands.

[0013] It is advisable to use the following concentration range of alloying elements, wt. %:

Iron	0.08 - 0.25
Zirconium	up to 0.26
Silicon	0.05 - 11.5
Magnesium	up to 0.6
Strontium	up to 0.02

Detailed description of the invention

[0014] The rationale for the proposed process parameters of the method for producing deformed semi-finished products from this alloy is given below.

[0015] Depending on the requirements for the final characteristics, the melt will contain iron and at least an element of the group consisting of Zr, Si, Mg, Ni, and Sc, in particular:

a) iron and at least an element of the group consisting of zirconium and scandium are used to produce deformed heat-resistant semi-finished products (with an operating temperature of up to 300°C);

b) iron, silicon and magnesium are used to produce deformed semi-finished products with high strength properties (not less than 300 MPa);

c) iron and at least an element of the group consisting of silicon, zirconium, manganese, silicon, strontium and scandium are used to produce welding wire;

d) iron and at least an element of the group consisting of nickel, copper and silicon are used to produce thin wire.

[0016] It is well known that the size of the structural constituents of casting bars is directly dependent on the cooling rate in the crystallisation interval, in particular, the size of the dendritic cell, eutectic components, etc. Therefore, a decrease in the crystallisation rate, at which the formation of a dendritic cell of less than 60 µm might lead to the formation of coarse phases of eutectic origin, will impair the processability during subsequent deformation processing resulting in a decrease in the overall level of mechanical characteristics on thin deformed semi-finished products (in particular, on thin wire and thin shapes). In addition, a decrease in the cooling rate below the required one will not ensure the formation of a supersaturated solid solution during the crystallisation of the casting bar, in particular, in terms of zirconium content, which will negatively affect the final physical and mechanical characteristics of the deformed semi-finished products.

[0017] If the rolling temperature of the initial casting bar exceeds 550°C, dynamic recrystallisation processes may occur in the wrought alloy, which may adversely affect the overall strength characteristics of the semi-finished product produced for further use.

[0018] For wrought alloys containing zirconium, the initial casting bar temperature should not exceed 450°C, otherwise coarse secondary precipitates of the Al₃Zr (Ll₂) phase or coarse secondary precipitates of the Al₃Zr(D0₂₃) phase may form in the structure.

[0019] If the press temperature of the rolled casting bar exceeds 520°C, dynamic recrystallisation processes may occur in the wrought alloy, which may adversely affect the overall strength characteristics. If the press temperature of the rolled casting bar is below 400°C, semi-finished products may exhibit worse processability when being pressed,

[0020] A decrease in the quenching temperature below 450°C will result in premature decomposition of the aluminium solid solution, which will adversely affect the final strength properties.

[0021] Examples of specific implementation of the proposed method are given below.

[0022] The method for producing casting bar affects the structure parameters for Al-Zr alloys and to a lesser extent for other systems. In particular, for Al-Zr alloys, all zirconium should be included into the aluminium solid solution, which is achieved by:

1) a rise in temperature above the liquidus for the Al-Zr system; and

2) a cooling rate during crystallisation.

[0023] Although it is almost impossible to measure the cooling rate directly in an industrial plant, the cooling rate has a direct correlation with the dendritic cell; for this purpose, this parameter is just introduced as a criterion.

Example 1

[0024] Under laboratory conditions, casting bars (with a cross-section area of 1,520 mm²) were produced from an Al-Zr type alloy containing 0.26% Zr, 0.24% Fe, and 0.06% Si (wt. %) under different conditions of crystallisation. The crystallisation conditions were varied by heating of the ingot mould. The casting temperature was 760°C for all options.

[0025] The structure of the casting bar and deformed rod with a diameter of 9.5 mm that were produced by rolling was studied using the metallographic analysis method (scanning electron microscopy). The initial casting bar temperature before rolling was 500°C. The measurement results are given in Table 1.

Table 1 - Effects of the cooling rate on the casting bar structure and the final size of Fe-containing phases of eutectic origin

No.	Cooling rate °C/s	Casting bar structure parameters
		Average dendritic cell size, µm	Structural constituents	Maximum transverse size of Fe-containing eutectic phases
1	3	98	(Al), Al3Zr (D023), Fe-containing eutectic phases	-*
2	5	85		-*
3	7	71	(Al), Fe-containing eutectic phases	3.8
4	11	60		3.1
5	27	45		2.5
6	76	29		1.6

(Al) - aluminium solid solution; Al3Zr(D023) - primary crystals of the Al3Zr phase with a D023 type of structure; * - failure to roll the casting bar due to the presence of primary crystals

[0026] According to the results given in Table 1, if the casting of casting bar is carried out at a cooling rate of 5°C/s and less, primary crystals of the Al₃Zr(D0₂₃) phase form in the Al-Zr alloy structure, which is an irremovable structural defect.

[0027] As can be seen from Table 1, it is only at a cooling rate of 7°C/s and higher in the crystallisation interval that the casting bar structure is an aluminium solid solution (Al), against which the ribs of Fe-containing eutectic phases with a size of 3.8 µm and less are distributed.

[0028] In order to assess the processability when deforming, wire rod with a diameter of 9.5 mm was produced from casting bar Nos 3-6 (Table 1), and thin wire with a diameter of 0.5 mm was produced from the wire rod. The results relating to the processability when drawing and the determination of the mechanical properties of the annealed wire are given in Table 2.

Table 2 - Mechanical properties of 0.5 mm diameter wire

No	σUTS, MPa	σ0.2, MPa	δ, %	Note
3	-	-	-	Low processability when drawing (breaks)
4	130	155	8	-
5	131	160	10	-
6	131	167	14	-

[0029] As can be seen from Table 2, high processability when drawing a thin wire with a diameter of 0.5 mm is ensured only at a cooling rate of 11°C/s and higher, at which eutectic particles of the Fe-containing phase form. High processability is provided by the achievement of the particle size of the Fe-containing phase, the maximum size of which does not exceed 3.1 µm.

Example 2

[0030] Deformed semi-finished products in the form of rods with a diameter of 12 mm were produced from an alloy containing 11.5% Si, 0.02% Sr, and 0.08% Fe (wt. %) by rolling and pressing successively.

[0031] The initial cross-sections of the casting bars were as follows: 1,080, 1,600, and 2,820 mm². The rolling of the casting bar and the pressing of the rolled casing bar were carried out at different temperatures. The rolling and pressing parameters are given in Table 3.

Table 3 - Rolling and pressing parameters for the Al-11.5% Si-0.02% Sr alloy

Casting bar cross-section mm2	Rolling			Pressing	Note
	Initial casting bar temperature °C	Final casting bar cross-section mm2	Degree of deformation in one pass when rolled, %	Degree of deformation when pressed %
1,080	450	340	56	76
	450	680	37	83
	450	960	11	88
1,600	450	340	70	-	Failure when rolled
	500	680	58	-	Failure when rolled
	500	960	40	88
2,820	500	340	83	-	Failure when rolled
	500	680	76	-	Failure when rolled
	500	960	66*	88

* - small cracks when rolled

Example 3

[0032] Rods were produced from an alloy containing Al-0.6% Mg-0.5% Si-0-25% Fe by various deformation operations: rolling, pressing, and a combined rolling and pressing process. Table 4 shows a comparative analysis of the mechanical properties (tensile strength). The cross-section of the initial casting bar was 960 mm². The rolling and pressing temperature was 450°C. The final diameter of the deformed rod was 10 mm. The tests were carried out after 48 hours of sample ageing. The design length in the tensile test was 200 mm.

Table 4 - Mechanical properties (tensile strength)

Deformation operation	σUTS, MPa	σ0.2, MPa	δ, %
Rolling	182	143	12
Pressing	151	123	25
Rolling and pressing	165	136	23

[0033] From the given results, it follows that the best percentages of elongation (δ) are achieved when the casting bar is pressed or pressed and rolled during the combined process. In this case, different percentages of elongation are achieved in the formation of a thin structure during rolling and pressing, in particular, a polygonised structure with an average subgrain size of not more than 150 forms after pressing, in contrast to rolling when the structure is mainly represented by a cellular structure.

Example 4

[0034] Rods were produced from alloys containing Al-0.45% Mg-0.4% Si-0.25% Fe (designation 1) and Al-0.6% Mg-0.6% Si-0.25% Fe (designation 2) (please refer to Table 5) by a combined rolling and pressing process in different modes. The rolling and pressing parameters are shown in Table 5. The cross-section of the initial casting bar was 960 mm². When rolled, the degree of deformation was 50%. When pressed, the degree of deformation was 80%. On leaving the pressing machine, the produced rods were intensively cooled with water to obtain a solid solution supersaturated with alloying elements. The cross-section of the initial casting bar was 960 mm². The rolling and pressing temperature varied in the range of 520-420°C, which made it possible to obtain different temperatures of the press-formed casting bar. When rolled and pressed, the temperature loss ranged from 20 to 40°C. The final diameter of the deformed rod was 10 mm. The tests were carried out after 48 hours of sample ageing. The design length in the tensile test was 200 mm.

[0035] Table 5 shows a comparative analysis of the percentage of elongation and electrical resistance. The specific electrical resistance values were indicative of the decomposition of the aluminium solid solution (32.5 ± 0.3 and 33.1 ± 0.3 µOhm^∗mm, respectively, correspond to the supersaturated condition for alloys 1 and 2 under consideration).

Table 5 - Percentage of elongation and electrical resistance according to the temperature of the rod after leaving the pressing machine

Designation	Rod temperature after leaving the pressing machine, °C	Specific electrical resistance of wire rod, µOhm/mm	Percentage of elongation, %
1	500	32.5	23.9
	450	32.5	23.7
	440	32.0	20.1
	430	31.5	18.1
2	500	33.1	23.9
	490	33.1	23.7
	470	32.6	20.1
	460	31.5	18.1
	400	31.1	17.1

[0036] From the results given in Table 5, it can be seen that a supersaturated solution can be obtained after pressing and intensive cooling with water, if the temperature of the initial casting bar is about 520°C and the temperature of the pressed casting bar is not lower than 490°C, which, in the case of quenching, provides for the possibility of achieving a supersaturated aluminium solution on the press-formed casting bar.

Example 5

[0037] A wire rod with a diameter of 9.5 mm was produced from technical-grade aluminium containing 0.24% Fe and 0.06% Si (wt. %) by a combined rolling and pressing process. The wire rod production process involved the following operations:

• continuous casting of the casting bar at a cooling rate providing the formation of a dendritic cell with an average size of about 30 µm. In this case, the casting bar structure was an aluminium solution, against which the eutectic ribs of the Fe-containing phase with a maximum size of not more than 1.5 µm were distributed.
• hot rolling at an initial casting bar temperature of about 400°C with a degree of deformation of 50%;
• subsequent pressing of the casting bar with a degree of deformation of 78% to produce a 15 mm rod
• subsequent rolling of the rod to produce a 9.5 mm wire rod.

[0038] Table 6 shows a comparative analysis of the mechanical properties (tensile strength) of the wire rod produced by the combined process and using conventional equipment for the continuous production of wire rod on the VNIIMETMASH casting and rolling machines.

Table 6 - Values of mechanical properties ensured by the combined rolling and pressing process and the VNIIMETMASH machine

Deformation operation	σUTS, MPa	δ, %
VNIIMETMASH	105	14.5
Rolling & pressing	108	20.5

[0039] The increased value of elongation of the casting bar produced by the combined method provides for 25% higher values of elongation in comparison with the conventional wire rod production method.

Example 6

[0040] A 3.2 mm diameter wire was produced from the 12 mm diameter rods that were produced using a combined rolling and pressing process. The initial casting bar cross-section was 1,520 mm². When rolled, the degree of deformation was 45%; when pressed, that was 86%. The resulting rods with a diameter of 12 mm were thermally processed at a temperature of 375°C for 150 hours and the wire was subsequently produced from such rods.

[0041] The loss of properties was evaluated after the one-hour-long annealing of the wire at a temperature of 400°C and calculated based on the ratio:

where

σ_initial - an initial ultimate strength of the wire

σ_anneal - an ultimate strength of the wire after its one-hour-long annealing at 400°C.

Table 7 - Effects of the parameters of the combined rolling and pressing of the Al-0.25% Zr alloy on the loss of properties of the wire after its one-hour-long annealing at 400°C

Casting bar temperature*, °C	Rod temperature after leaving the pressing machine*, °C	Loss of properties of the wire following its one-hour-long annealing at 400°C, %
520	500	12
500	480	9
470	450	8
420	400	8
360	340	6
320	300	9
300	270	12

* - During the production process, the casting bar temperature was maintained with an accuracy of 10°C.

[0042] From the results shown in Table 7, it can be seen that at a high temperature of the casting bar the loss of properties exceeds 12%, which is associated with an uncontrolled and uneven (fan-shaped) decomposition of the aluminium solid solution, including partial formation of the Al₃Zr phase already during the deformation processing. With the temperature being decreased, no uneven decomposition was observed. When the temperature fell below 300°C, the wire was characterised by higher ultimate tensile strength, which caused a greater decrease in the strength properties during annealing.

Claims

1. The method for producing deformed semi-finished products from an aluminium-based alloy, which comprises the following stages:

a) preparing a melt containing iron and at least an element selected from the group consisting of zirconium, silicon, magnesium, copper, and scandium;

b) producing a continuous casting bar by crystallisation of the melt at a cooling rate that provides the formation of a cast structure characterised by a dendritic cell size of not more than 60 µm;

c) producing a deformed semi-finished product of a final or intermediate cross-section by hot rolling of the casting bar, with an initial casting bar temperature being not higher than 520°C and a degree of deformation being of up to 60%, and additionally using at least one of the following operations:

- pressing of the casting bar in the temperature range of 300-500°C by passing of the casting bar through the die;

- water quenching of the resulting deformed semi-finished product at a temperature not lower than 450°C;
in this case, the deformed semi-finished product structure is an aluminium matrix with at least one selected alloying element and eutectic particles with a transverse size of not more than 3 µm that are distributed therein.

2. The method according to claim 1 characterised in that the rolling is carried out at room temperature.

3. The method according to claim 1 characterised in that the rolling of the press-formed product is carried out by passing it through a number of rolling mill stands.

4. The method according to claim 1 characterised in that the following concentration range of alloying elements, wt. %, is used:

Iron	0.08 - 0.25
Zirconium	up to 0.26
Silicon	0.05 - 11.5
Magnesium	up to 0.6
Strontium	up to 0.02

5. The method according to claim 1 characterised in that iron and at least an element of the group consisting of zirconium and scandium are used in the melt to produce deformed heat-resistant semi-finished products, with an operating temperature being of up to 300°C.

6. The method according to claim 1 characterised in that iron, silicon and magnesium are used in the melt to produce deformed semi-finished products with high strength properties (not less than 300 MPa);

7. The method according to claim 1 characterised in that iron and at least an element of the group consisting of silicon, zirconium, manganese, silicon, strontium, and scandium are used in the melt to produce welding wire.

8. The method according to claim 1 characterised in that iron and at least an element of the group consisting of nickel, copper, and silicon are used in the melt to produce thin wire.